248 Plant Physiology 



as controls. Place the pots in the greenhouse upon fresh cinders, 

 separating the two lots and protecting the pots as well as practi- 

 cable from contamination by dust or soil from other pots or beds. 

 Water with distilled water only. After the plants have grown 

 sufficiently, compare the two lots, as indicated, for nodule for- 

 mation and amount of growth. 



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